1. Field of the Invention
The present invention relates to rubber compositions having good metal adhesive properties, and more particularly, to rubber compositions suitable for adhering to metal which exhibit an excellent adhesion when used as a steel cord coating rubber.
2. Description of the Prior Art
Recently, a remarkable development has been attained in a so-called "steel tire" wherein steel cords are used for belts or carcass plies thereof, while diversified studies have been made of adhesion between steel cord and rubber.
In general, methods for bonding a steel cord with a rubber can be roughly classified into two groups: one of those known as an RHS bonding system which utilizes resorcinol/hexamethylene tetramine/silica, and another one wherein metal salts, particularly, cobalt salts of organic acids are compounded with the rubber. Specifically the latter, inter alia, has been widely utilized and rubber compositions compounded with metal salts have heretofore been proposed as a steel cord coating rubber. For example, in Japanese Patent Application Publication No. 56-39,828, Japanese Patent Application Laid-open No. 54-52,188, Japanese Patent Application Publications Nos. 50-33,905, 49-20,072, 52-26,276 and 54-31,511, Japanese Patent Application Laid-open No. 58-89,631, etc., there are disclosed rubber compositions wherein cobalt naphthenate, cobalt stearate, cobalt oleate, cobalt tallate, cobalt resinate or the like is compounded as an adhesion promoter.
Further, Japanese Patent Application Publication No. 56-19,820 discloses a rubber composition compounded with cobalt acetylacetonate.
Meanwhile, demands for safety, high speed traveling properties, durabilities, etc. of automotive tires have recently been more and more increasing so that the above conventional techniques do not always provide an adhesion satisfactory enough.
Namely, in tires reinforced with steel cords, heat generated during running induces deterioration of bonding force between the steel cords and rubber, causing a fatal break-down of the tires. However, according to the conventional techniques, a satisfactory thermoresistant adhesion is not obtainable.
Alternatively, from the standpoint of rationalization and energy saving in manufacturing processes, a rapid vulcanization is material. Therefore, adhesion promoters have become required for promoting adhesion of rubbers to steel cords not only to improve the thermoresistant adhesion but also to augment an initial bonding strength. However, adhesion promoters to meet these required characteristics have never been proposed.
The cobalt acetylacetonate compounded rubber compositions described in Japanese Patent Application Publication No. 56-19,820 can provide relatively a high bonding strength. However, these rubber compositions have such drawbacks as a low vulcanizing rate and an offensive smell of unvulcanized rubber compositions.